Dendritic cells (DC's) are a new cell type in lymphoid organs that have been studied primarily in mice, and more recently in man. Mouse DC's can be purified in good yield, and maintained in tissue culture for several days with little change in their appearance and properties. DC's have a distinctive surface topography by scanning electron microscopy and they express Ia-alloantigens (I region associated). The most striking functional property of DC's is that they are the most potent stimulator of transplantation reactions in tissue culture, yet to be identified. Preliminary evidence has been obtained that they are potent accessory cells in normal immune responses as well. This project has 2 general aims. The first is to continue to characterize purified DC's including biochemical studies on transplantation antigens, as well as functional studies in several immune responses. The second aim is to further distinguish DC's from macrophages, another cell type with which DC's have previously been confused. One major approach will be to develop specific monoclonal antibodies that distinguish DC's and macrophages.